Terracotta is a lovely and subtle reddish hue. Real terracotta is made typically from red clay. While the red is a rich hue, it is also tempered, as it is an earth tone, with some neutral components. To mix colours for terracotta, begin with an earthy red and some white to lighten it a touch. You should also have a brown earth colour, like burnt sienna on-hand, to darken it to your taste or to render a good deep tone for a terracotta in shadow. You can use the final hues to either render a terracotta floor in an interior, or as a rich neutral in another part of your painting. Zinc white produces a cleaner tone than other whites, but you may substitute titanium white or other permanent white if you wish.
Place a teaspoon of Venetian red, zinc white and burnt sienna on your palette.
Scoop equal amounts of zinc white and Venetian red into the centre of the palette.
Blend the Venetian red and zinc white together with sweeps of the palette knife. Mix until there is no broken colour (the colour is fully consistent, with no streaks).
Check your colour to see if it's terracotta red. If it's too light, add Venetian red. If it's too red, add zinc white. Use a tiny bit at time when adjusting.
Continue until the terracotta red matches your terracotta subject (or desired hue).
Scoop a portion of the finished colour to a different area of the palette. Add a speck of burnt sienna. Mix well. This produces a slightly darker rendition of the terracotta hue for use in shadow or other area. Add a speck more burnt sienna for a darker hue.